Because a large number of 401k plans are drawing upon stocks so that retired folk can live out their retirement. The boomers are retiring, and they're the first major group to retire under 401(k) program. They will be selling stocks and bonds (and whatever else they've invested into) and are no longer buying stocks with their end-of-career top-level paychecks.
Does anyone have an actual breakdown of how much the younger generations are placing away in a 401k - if that's even possible?
As you'd expect the contribution percentage increases with age (and the amount presumably increases even more). But even under 25s are at about 5% which is more than I would have expected though this is presumably just among people who have the option.